1. Field of the Invention
This application is related generally to video display terminals and more specifically to apparatus for generating a plurality of horizontal and vertical timing and synchronization signals for a bit-mapped raster scan terminal.
2. Description of the Prior Art
The picture displayed on a video monitor screen can be considered to consist of a large number of generally horizontal, parallel "raster lines" or lines of displayed information. As the monitor's electron beam scans along each line, location on the face of the cathode ray tube are stimulated. Each such location is commonly termed a pixel and the resolution of the screen is specified as the number of lines displayed multiplied by the number of pixels on each line. In a color monitor, the colors are generated by differentially activating red, blue and green electron guns which are aimed at three very close, but not coincident, points on the screen face.
Raster-scanned terminals are typically designed to display either a non-interlaced or an interlaced raster. In a non-interlaced terminal every raster line on the screen is scanned during every "frame" or vertical deflection cycle. Each frame is therefore a complete picture. In an interlaced terminal, every other raster line is scanned during each frame (either the "odd" lines or the "even" lines). It therefore takes two frames to make up a complete picture.
Normally, either 50 or 60 frames are completed per second, depending on the external electrical power available to the terminal. At 60 frames per second, the non-interlaced screen will be refreshed 60 times per second while the interlaced screen will be refreshed 30 times per second. A standard non-interlaced frame is commonly defined as having 525 raster lines. The actual number of lines of information which is displayed is somewhat less since some of the 525 are taken up by the retrace process. 525 lines per frame and 60 frames per second equals 31,500 lines per second or 31.5 KHz. The interlaced terminal, therefore, operates at one-half of this rate or 15.75 KHz. Since monitors commonly operate in only a single mode the timing and synchronization logic in prior art terminals typically is designed to operate only in an interlaced or a non-interlaced mode.
Raster scan graphics terminals are typically designed as either character or bit-mapped terminals. In character graphics, the displayed picture is made up of a combination of predefined alphanumeric characters and simple shapes, with each character or shape being a matrix of pixels (e.g. 8.times.10) which has a single address in terminal memory. In bit-mapped graphics, every pixel is associated with one or more locations in memory. In a monochrome terminal, a single bit per pixel would be adequate to describe the pixel state. In a color terminal, however, multiple bits per pixel are required. For example, if it is desired to be able to display up to 16 different colors on the screen simultaneously, each pixel must have four bits associated with it to define its color.